There's no opApply overload taking an invariant(char)[]. You can use
dmd 1.x compilers until things have settled down in 2.x. Or you just
use a char[] instead of the string alias. If you need the string to be
invariant you can cast it to string before using it.
According to the docs for InputStream, opApply is allowed to reuse the
char buffer, so depending on your needs, you might have to use .idup or
.dup.